This invention regards a method of filtering pump noise. More specifically, it regards a method of eliminating or reducing pump generated noise in a telemetry signal transmitted via the fluid exiting from the pump, by using the instantaneously measured angular position of the pump as a fundamental variable in an adaptive mathematical noise model.
In this context, pump generated noise, pump noise or pressure noise mean measurement or test signals that can be attributed to the pressure fluctuations in the pumped fluid. The angular position of the pump means the angular position of the pump crankshaft or actuating cam axle.
Drilling fluid pulse telemetry is still the most commonly used method of transmitting downhole information to the is surface when drilling in the ground. A downhole telemetry unit, which is normally located in a drill string near the drill bit, measures parameters near the drill bit and encodes the information into positive and negative pressure pulses. These pressure pulses propagate through the drilling fluid in the drill string and on to the surface, where they are picked up by one or more pressure sensors and decoded.
Generally the pressure pulses will attenuate on their way up through the drill string, and the attenuation increases with frequency and transmission distance. In long wells therefore, the telemetry signal may become so weak as to make decoding difficult. Thus the pump generated pressure noise, which often contains components in the same frequency range as that of the telemetry signal, is a factor that limits the quality and rate of the data transmission. Thus reducing or eliminating pump noise is vital to allow the telemetry data rate to be increased.
Pump noise may be reduced mechanically by means of e.g. a pulsation moderator, or electronically by filtration of the measured pressure signal. The first method is not very suitable, as it also dampens the telemetry signal in addition to dampening the pump noise. Moreover, mechanical dampers represent undesirable costs.
Prior art comprises a variety of methods of filtering out pump noise. Many of these techniques describe methods which use more than one sensed pressure signal. It may for instance be a case of pressure signals sensed in several locations in the installation, or complementary flow rate measurements.
A characteristic of these known methods is the fact that the pump noise is related to time.
U.S. Pat. No. 5,146,433 describes a method in which the pump noise is related to the linear position of the pump piston. The piston position is measured by a so-called LVDT sensor. According to this method calibration must be carried out when there is no pulse telemetry signal present. These conditions represent significant disadvantages because the linear position of the piston does not fully define the angular position of the pump, and because many pulse telemetry systems can not be stopped after the drilling fluid rate has exceeded a certain level. Furthermore, the periods in which telemetry signals are transmitted may be of such a long duration that the drilling conditions and noise picture undergo significant changes. As an example, a valve may start to leak, whereby the noise picture will undergo a dramatic change, making the statically calibrated noise picture irrelevant.